So, Paul Sou asks for the tablets to be removed as soon as possible and these tablets are described as tum Reguli, tum Praefectorum, in honorem supradicti Domini Pauli, quatenus mathematici,"both of the viceroy and the Prefects, in honor of the aforementioned priest Paul, quatenus mathematician,...".

It seems to me that here quatenus doesn't introduce a clause and is used as an adverb. It could mean something like "until then mathematician" > "at the time (the tablets were made)" but I can't find any parallel for such a use of quatenus...Or that the tablets were made in his honor "in so far as he is mathematician" but then why the genitive, mathematici ?

Looks as if your last suggestion is right, “insofar as he’s mathematicus,” “as being a mathematician,” with quatenus no longer introducing a clause but used more like utpote, or Greek ἅτε or ὡς. Doesn’t seem too drastic a shift. Kind of in line with adverbial quantocius, equally unclassical?

What’s meant by mathematicus anyway? Just someone with learning, who could read the tabulae hung in the church hall in his honour?? The tabulae will be wooden(?) writing boards, hung on the wall like scrolls (which would be cartae?)? I'm assuming pien is 4th tone rather than 1st. (Just showing off there.)

Here, from today's New York Times, is an example of "as far as" used like your priest's quatenus, without a verb.

“We’re trying to verify the pics, and I am not convinced they are authentic,” said Erin Evers, the Human Rights Watch researcher in Iraq. “As far as ISIS claiming it has killed 1,700 people and publishing horrific photos to support that claim, it is unfortunately in keeping with their pattern of commission of atrocities, and obviously intended to further fuel sectarian war.”

I'm sorry to treat this news-piece in the same spirit as the English schoolmaster treated Sophocles' Oedipus at Colonus, as "a veritable treasure-house of grammatical peculiarities."